1. Field of the Invention
This invention relates generally to optical communication links; and more particularly to phase modulation in input and reference optical paths, yielding an ultrahigh-fidelity output signal at a difference frequency of the input and reference signals.
Communication links of greatest interest in this field are directed primarily (but not exclusively) to carrying microwave- or radio-frequency data. Digital data may be of principal interest, but analog signals too can be carried on remote links of the type under consideration.
Such data may include several or very many microwave-frequency or RF data signals multiplexed on a single optical carrier. In some cases these individual data signals in turn may represent several or very many lower-frequency data signals, extending even to inclusion of audio signals and other signals in like frequency range.
2. Prior Art
In most telecommunications systems, a design goal is to develop as many channels, and as much data in each channel, as possible--using the smallest possible number of optical fibers or other optical paths. Where fidelity is of utmost importance, however, some known systems dedicate a parallel optical channel for developing a verifying signal of some sort.
Some prior communication systems outside the fiberoptic area have featured mixing or heterodyning the data with a reference signal, and in some cases transmitting the latter along the parallel channel. These systems have suffered from a perceived necessity to place the reference source at the transmission end of the system, where it is unavailable to the data user for adjustment. That limitation forecloses convenient tuning of the reference signal to optimize the replication of the particular data signal being transmitted.
These prior systems suffer also from susceptibility of the reference signal to degradation in transmission--making it unavailable in pure form at the reception end of the system for remixing. In some cases it has been seen as best to regenerate an equivalent reference signal in a local oscillator at the reception location, for use in remixing, but this approach has led to severe problems of frequency drift, frequency error and excess phase noise.
Prior fiberoptic communications strategies generally involve modulating an optical source with the data--for example, pulsing a laser under control of the data signal. Sources amenable to such modulation are subject to adverse power limitations, relatively high noise levels, or both.
Further, generally in prior fiberoptic communication systems the optical detector must follow a high-frequency data signal. With microwave-frequency data this condition requires very fast detectors, again leading to compromises in noise level, linearity and other detector parameters.
U.S. Pat. Nos. 4,390,974 to Siems, 4,420,260 to Martinelli, and 4,533,247 to Epworth are optical remote transmission systems using interferometric principles to enhance performance. Other United States Patents that exemplify the state of the art in interferometric modulators, demodulators, error correction, and related areas include 3,906,401 to Seidel, 4,709,978 to Jackel, 4,265,534 to Remijan, 4,721,385 to Jelalian, 4,730,171 to So, 4,568,408 and 4,360,272 to Schmadel, and 4,773,758 to Shaw.
Certain fiberoptic sensors, configured as Mach-Zehnder interferometers, bear some very superficial resemblance to the optical system of my invention. As will be more readily appreciated after studying the description of my invention which follows, they differ in several crucial and fundamental ways. In particular, the prior interferometric sensors generally have only one modulator, do not employ any beating (i.e., difference-frequency operation), and operate in a linear part of the interferometer transfer function.
In radio technology, some of the aspects of my invention that will be described below have conceptual analogs or counterparts that are known. As far as I am aware, however, heretofore such analogs have not been suggested for use in optical links. Even given such a suggestion, persons of ordinary skill would not perceive how to implement such analogs in optical hardware.